This invention relates to a telemetry system for implantable devices. Such devices are used for a wide range of purposes within the body. The most commonly known of such devices is the cardiac pacemaker. Other well-known devices are for stimulation or sensing, or both, with respect to the brain, spinal cord, muscles, bones, nerves, glands or other body organs or tissue.
Implantable devices are becoming more and more complex and commonly include sophisticated data processing hardware such as microprocessors, or devices akin to microprocessors, ROM and RAM memories, LSI (large scale integration) devices as well as other computer hardware and related techniques. Information transmitted to and from the implanted device comprises device identification, biological data, parameters of present operation of the device (from previous settings), technical information concerning proper functioning of the device, battery condition, patient and physician data, up-to-date programming information for the device and verification of information transmitted to and from the device.
With more and more data being processed and available within the implantable device, there is a need to transmit more and more data from the implanted device to external devices, for analysis, reprogramming of the implantable device or other purposes. The need is for data to be transmitted in both directions in a reasonable amount of time.
There are, of course, limitations surrounding the design of new implantable systems of portions thereof. One of the most limiting aspects is the conservation of energy. An implanted device is customarily powered by a long-lasting internal battery and so the current consumption of a telemetry system becomes one of the most important of factors to be considered in the design of such a system. It is now possible to include A to D converters (having low energy consumption) in implantable devices inasmuch as they can now readily be incorporated in LSI circuits. Such converters draw little current while converting analog signals to digital signals.
Prior art devices have utilized various methods of communicating non-invasively through the skin. U.S. Pat. No. 4,223,679 entitled Telemetry Means for Tissue Stimulator System, issued Sept. 23, 1980 shows a device which uses little or no current to transmit information, relying on reflected impedance of an internal L-R or L-C circuit energized by an inductively coupled, external L-R or L-C circuit. The external circuit may utilize an RF (radio frequency) magnetic field carrier. In the cited patent, a voltage controlled oscillator (VCO), in the implanted device, is controlled by the signal to be telemetered. The VCO, in turn, varies the impedance which is reflected. If the signal controlling the VCO is a binary digital signal (varies, from one value to a single other value) it becomes encoded by the VCO which then varies from one frequency to one other frequency. This technique is known as FSK (frequency shift keying). Each bit duration--the time in which the binary digit (bit) is expressed--requires a number of carrier cycles. Therefore, the bit rate cannot be higher than 10 to 30 per cent of the VCO center frequency. On the other hand, the RF carrier frequency cannot be too high inasmuch the implanted device metal enclosure attenuates as a one-pole low-pass filter having an upper cut-off frequency at 10 to 30 kHz. Also, the external oscillator L-C circuit usually has a Q (quality factor) of 20 to 50, which limits the useful modulation bandwidth to 2 to 5 per cent of the RF carrier frequency. Using FSK encoding, the maximum bit rate is thus limited to 0.2 to 1.5 per cent of the RF carrier frequency. Thus, a 36 kHz carrier frequency could transmit 72 to 540 bits per second. Data rates are commonly on the order of 100 bits per second or less. With thousands and even tens of thousands of bits of data to be transmitted, much higher telemetry speed is needed. The information can then be read out in a fraction of the time formerly required.
Another prior art device is illustrated by U.S. Pat. No. 4,281,664 issued Aug. 4, 1981, for an Implantable Telemetry Transmission System for Analog and Digital Data. The system therein uses a VCO to FM encode the analog and digital data in the same way as the first mentioned patent. It pulses an implanted coil on each positive transition of the VCO. At each pulse, the above coil will generate a damped oscillation whose frequency is determined by a parallel capacitor. At the receiving side, another coil, tuned to the above frequency, receives the damped pulses and receiving circuitry reconstructs the VCO positive-going transitions. Digital data is transmitted by FSK using the VCO in a similar manner as explained therein for analog data. Such scheme has the drawback that the bit rate cannot exceed approximately 1 per cent of the carrier frequency. Inasmuch as the implanted metal enclosure attenuates the high frequency, the carrier frequency cannot be increased above approximately 10 kHz without a corresponding increase in coil driving power, which in turn, will require increased power consumption.
The implanted device tranmitter, in accordance with one emobidment of the present invention, encodes serial data to be transmitted using phase-shift-keying (PSK, or Manchester encoding) on a fixed frequency clock. The resulting signal controls an on/off switch (such as a semiconductor switch) connected across an L-C circuit tuned to the external device RF carrier frequency. The inductor (providing the L) is magnetically coupled to an external inductor (which provides the L in an external L-C circuit.) As the switch in the implanted device is opened and closed in accordance with the data to be transmitted, it causes a variation (modulation) of the reflected impedance which is received by the external L-C circuit. It may be that the switch, which is electronic in nature, exhibits little or no resistance (or impedance) when closed and a greater amount of resistance (or impedance) when in the open condition. The concept is that the impedance, or characteristics of the L-C circuit be varied, in response to the signal to be transitted. In other embodiments, the electrical charge in such circuit may be provided from within the implanted device or, an rf transmitter utilizing the L-C circuit as its antenna may be used to transmit information.
In a first mode of operation, the sytem of the invention permits transmitting digital data without the need of a VCO in the transmitter, if desired, and allows a higher bit rate with minimal hardware implementation. The bit rate can now be 2 to 5 per cent of the RF carrier frequency, or 3.3 to 10 times faster than the system discussed above.
In an alternate embodiment, a second mode is provided in which the implanted system uses analog signals required to be telemetered to modulate a voltage-controlled oscillator (VCO) providing frequency modulated signals which can also be transmitted by the telemetry system.
In another embodiment, a third mode is provided in which the implanted system in the sending of analog signals (which causes frequency modulated signals) inserts from time to time digital signals which may comprise a few bits of digital information or a "marker"
In still another embodiment, a fourth mode is provided wherein, instead of using a fixed frequency clock as the carrier for the PSK transmission of digital data, modulated FM carrier is used as the clock (and carrier) for the digital signals, and thus combines in one channel simultaneous transmission of both analog and digital information.
A further variation to the above embodiments is one in which, in place of the reflected impedance principle, the implanted coil (the coil tuned to a 5 to 50 kHz frequency by a capacitor,) is excited by a series of impulses corresponding to the positive edges of the encoded data, as taught in FIGS. 13 and 14.
The antenna for transmission from the implanted device, at higher frequencies, becomes very short and may, at such high frequencies, provide the necessary inductance and capacitance itself. A tuned stub may be satisfactorily used in some instances. Ferrite beads, ferrite coils, toroids and other high frequency components may be utilized in the transmission and reception portions or in the antennas in order to obtain proper impedances and to match such impedances.
It is possible using the concept of the invention to transmit at frequencies up to, and possibly beyond, 200 MHz. Above such frequencies, the saline solution of the body tissue begins to cause problems such as RF heating, and energy losses, including transmission losses.
Still another variation is one in which an active rf transmitter is used in the implanted device to transmit the information, using the implanted coil as the antenna.
It is intended that all suitable forms of transmission, whether active or passive, be included within the concept of the invention. Thus, reflection of impedance, transponder, rf transmission (or other active transmitter), telemetry, current or voltage variation, ultrasonic, light, infrared, or other suitable method or mode of getting information from one point to another without a direct wire are within the concept of the invention.
Thus, the words "transmission", "obtaining" or "providing" are to be interpreted herein and in the claims as including within their meaning all such methods or modes.
In addition to PSK encoding as explained herein, a number of other pulse encoding schemes may be used, such as FM (frequency modulation--1 pulse for a zero, 2 pulses for a 1), MFM (modified FM--using less than one pulse per bit--known as Miller coding) or MM (modified Miller) as well as various biphase coding schemes. Such encoding schemes are known to those skilled in the encoding art. Any one of such encoding schemes will permit the same increase in data rate as explained in the above PSK encoding as explained in connection with PSK encoding, without extra power to be required. This will become apparent in connection with FIG. 13 and its description.
The signal received b the external L-C circuit is processed and decoded from PSK (or other encoding as previously mentioned) to NRZ data. For the embodiment in which FM modulation (VCO) is used to transmit analog data in addition to digital data, the output for the analog data is taken from an FM demodulator and filter.
Suitable switches in the implantable device provide selection of the available modes. Such switches may, of course, be controlled, or set, by information transmitted from outside the patient's body or from internal control.
A similar transmitter and receiver may be used to transmit information from outside the body to the implanted device. It is most likely that the information transmitted to the implanted device would be digital in nature, however, both analog and digital data could be transmitted in, just as both kinds of data can be transmitted out. Some of the components, viz., the L-C circuits of the outward transmitting system could be shared and used in transmitting information into the implanted device.